Information Technology Reference
In-Depth Information
At the beginning of this section, it was mentioned that suboptimal routing is quite possible
with this architecture. This is because the path selection outlined in Table 5-2 is the same
regardless of link speeds. If all links in the core are the same bandwidth, a satisfactory path
selection is achieved. However, if the links between R11 and R4 via R3 are all DS3s, and
the links between R11 and R6 via R8 and R9 are OC-12s, for example, there is the potential
for poor use of available bandwidth.
Assume that the path from R11 to R9 is preferable to the path from R11 to R3 for traffic
destined for 10.2.0.0/16. This can be accomplished by manually applying an inbound route
map on R11 to the eBGP session with R9 that changes the local preference attribute to
prefer prefixes received via AS 65103. The configuration is shown in Example 5-6, and the
result is shown in Example 5-7.
BGP Configuration Preferring Prefixes via AS 65103
Example 5-6
router bgp 65104
no synchronization
bgp log-neighbor-changes
network 10.4.0.0 mask 255.255.0.0
neighbor 172.16.13.9 remote-as 65101
neighbor 172.16.13.17 remote-as 65103
neighbor 172.16.13.17 route-map LPREF in
no auto-summary
!
route-map LPREF permit 10
set local-preference 120
!
The same result can be achieved in this scenario by applying a higher WEIGHT to prefixes
received from R9, because AS 65104 has only one BGP speaking router.
NOTE
Prefix 10.2.0.0/16 with Modified LOCAL_PREF
Example 5-7
R11#show ip bgp 10.2.0.0
BGP routing table entry for 10.2.0.0/16, version 20
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Flag: 0x208
Advertised to non peer-group peers:
172.16.13.17
65103 65102
172.16.13.17 from 172.16.13.17 (172.16.9.1)
Origin IGP, localpref 120, valid, external, best
65101 65102
172.16.13.9 from 172.16.13.9 (172.16.3.1)
Origin IGP, localpref 100, valid, external
120
best
100
